1. Field of the Invention
This invention relates to communication networks and more particularly to communication networks using individually switched small blocks of data for communication through the network.
2. Discussion of the Prior Art
There exists today a great demand for rapid and efficient communication of digital data through a network. Rapid response is particularly required for such applications as interactive communication between a plurality of terminals and a host data processor as in an educational terminal system. Such a system is particularly demanding because of the nature of the interactive communication between a terminal and a host computer. A terminal keystroke may require a host response in which enough data is communicated back to the terminal to fill a CRT display. If even short, perceptible delays occur in the communication of this information, the terminal user may become discouraged and the value of the system is impaired.
While a number of data communication systems are currently known, they fail to provide the combination of reliability, economy and rapid response that is available with the present system. The most straight forward approach to data communication is to provide a dedicated communication link between each terminal and the host. The link can be provided either continuously or only upon demand while a terminal is operational. Such an arrangement solves the delay problem because the link is always available for the communication of data. However, a link that is capable of carrying high speed digital data is quite expensive and the utilization of a dedicated data link is very poor because data is actually being sent to or from an operational terminal only a small percentage of the time. Where any substantial distances are involved the costs can be prohibitive for many network applications, including educational terminals.
An approach which helps to reduce the cost of a communication link is to employ switching to permit sharing of a link. A complete communication link is established between a sender and a recipient of data, but only during the times that data is actually being transmitted. Such a system still remains quite expensive and cannot accommodate well multiple simultaneous requests for the data link during times of peak demand.
Further data link efficiencies are realized by a technique known as packet switching in which a block of data, typically having 1024 bits or more is sent as an individual entity. The blocks are sent through the network from switching point to switching point and different portions of a data link can carry different data blocks simultaneously. There is no need to establish at one time a dedicated path from sender to receiver and blocks relating to different senders and recipients can be interspersed on a shared data link segment to maximize the efficiency of use. However, substantial communication data or "overhead" is required just to direct the data blocks through the switching network and the delay between sending and receipt may be substantial from the viewpoint of an interactive terminal user. In addition, the block or packet switching sytem does not readily accommodate peak demands in which communication demand exceeds the capacity of the data links. Such a system is discussed in articles by Lawrence G. Roberts, "Data by the Packet," Spectrum pages 46-51, Vol. 11, No. 2 (February 1974 ) and by R. E. Kahn, "Resource-Sharing Computer Communication Networks," Proceedings of the IEEE, pages 1397-1407, Vol. 60, No. 11 (November 1972).
One arrangement using time division multiplexing with dedicated time slots is shown in U.S. Pat. No. 4,007,441. Small, 4 element communication rings are established with communication processor coupling the small rings to terminal devices or other rings.
Still another arrangement that is commonly known as a Pierce Ring is illustrated in U.S. Pat. No. Re. 28,811 and described in an article Pierce, J. R. Coker, C. H., and Kroppl, W. J., "Network for Block Switching of Data" IEEE Conv. Rec., New York, March 1971. Another loop arrangement is taught by Fraser, "Digital Data Communication System," U.S. Pat. No. 3,749,845. In this arrangement a series of interconnected loops of different capabilities interconnect the devices of a network. Such a system is further discussed in an article by Hayes, J. F. and D. N. Sherman, "Traffic Analysis of a Ring Switched Data Transmission System", The Bell System Technical Journal, Vol. 50, No. 9 Pages 2947-2978 (November 1971).
Further background on data communications can be obtained from Proceedings of the IEEE, Vol. 60, No. 11, November 1972, which is a special issue on computer communications.
Additional background information is available from Hayes, J. F. and D. N. Sherman, "A Study of Data Multiplexing Techniques and Delay Performance", The Bell System Technical Journal, pages 1983-2011, Vol. 51, No. 9, (November 1972) and from "Computer Network Requirements Study" prepared by Network Analysis Corporation, Beechwood, Old Tappan Road, Glen Cove, N.Y. in March 1974.